Southern Ocean

The Southern Ocean, sometimes called the Antarctic Ocean, is the world’s southernmost body of water, generally defined as the waters south of the 60th parallel that encircle Antarctica. It covers 21,960,000 square kilometers (8,480,000 square miles), making it the second smallest of the five main oceans—larger than the Arctic but smaller than the Pacific, Atlantic, and Indian. Its deepest point was measured in early February 2019 by the Five Deeps Expedition using multibeam sonar, reaching 7,434 meters (24,390 feet) at coordinates 60° 28' 46"S, 025° 32' 32"W. Expedition leader Victor Vescovo, who piloted the submersible DSV *Limiting Factor* to the bottom on February 3, 2019, has proposed naming this spot the "Factorian Deep."

For centuries, geographers disagreed on whether the Southern Ocean should be defined by the seasonally shifting Antarctic Convergence—where cold Antarctic waters meet warmer Subantarctic waters—or whether its waters should simply be considered the southern extensions of the Pacific, Atlantic, and Indian Oceans. In 2000, the International Hydrographic Organization (IHO) settled the debate after recognizing the importance of the Southern Ocean overturning circulation, and now defines the ocean as the waters south of that circulation’s northern limit. This overturning circulation forms the second half of the global thermohaline circulation, after the Atlantic meridional overturning circulation (AMOC). Like AMOC, it has been heavily affected by climate change, increasing ocean stratification and potentially slowing or even collapsing past a tipping point. Such a collapse would harm global weather and marine ecosystems over centuries, and ongoing warming is already altering those ecosystems.

**Definition and term use**

The IHO’s precursor, the International Hydrographic Bureau, first agreed on ocean borders and names at a conference on July 24, 1919, publishing them in *Limits of Oceans and Seas* in 1928. Since then, the Southern Ocean’s limits have moved progressively southward; after 1953, it was omitted from the official publication, leaving local hydrographic offices to set their own boundaries. The IHO included the ocean in its 2000 revisions, defining it as waters south of the 60th parallel, but this has not been formally adopted due to unresolved disputes, such as the naming of the Sea of Japan. The 2000 definition was circulated as a draft in 2002 and is used by some IHO members and organizations like the CIA World Factbook and Merriam-Webster. The Australian government considers the Southern Ocean to lie immediately south of Australia. The National Geographic Society officially recognized the ocean in June 2021; before that, it showed the Pacific, Atlantic, and Indian Oceans extending to Antarctica on its maps. Map publishers Hema Maps and GeoNova use the term on their maps.

**Pre-20th century**

"Southern Ocean" was once an obsolete name for the Pacific Ocean or South Pacific, coined by Spanish explorer Vasco Núñez de Balboa, the first European to see the Pacific from Panama. "South Seas" is a less archaic synonym. A 1745 British Act of Parliament offered a prize for discovering a Northwest Passage to "the Western and Southern Ocean of America." Authors using "Southern Ocean" for waters around the unknown southern polar region used varying limits. James Cook’s second voyage account implies New Caledonia borders it. Peacock’s 1795 *Geographical Dictionary* placed it "to the southward of America and Africa"; John Payne in 1796 set the northern limit at 40 degrees; the 1827 *Edinburgh Gazetteer* used 50 degrees. *The Family Magazine* in 1835 split the "Great Southern Ocean" into the "Southern Ocean" and the "Antarctick [sic] Ocean" along the Antarctic Circle, with the Southern Ocean’s northern limit connecting Cape Horn, the Cape of Good Hope, Van Diemen’s Land, and southern New Zealand. The United Kingdom’s South Australia Act 1834 described the waters forming the southern limit of the new province as "the Southern Ocean." The Colony of Victoria’s Legislative Council Act 1881 delimited part of Bairnsdale as "along the New South Wales boundary to the Southern ocean."

**1928 delineation**

In the 1928 first edition of *Limits of Oceans and Seas*, the Southern Ocean was defined by land boundaries: Antarctica to the south, and South America, Africa, Australia, and Broughton Island, New Zealand to the north. The detailed limits ran from Cape Horn in Chile east to Cape Agulhas in Africa, then east to the southern coast of mainland Australia to Cape Leeuwin, Western Australia. From there, the limit followed the coast east to Cape Otway, Victoria, then south across Bass Strait to Cape Wickham on King Island, along the island’s west coast, and then the remainder of the boundary.